In many applications in the field of hydraulics a pair of hydraulic rams are used to cause displacement of a load. It is often important that the rams have a known, and relatively equal displacement, even if the load is placed more toward one ram than the other. A typical application in which this situation arises occurs in the use of twin post lifts for lifting vehicles in a repair shop. It is not desirable to lift one side of a vehicle significantly further than the other. A general solution to this problem is to employ a master and slave cylinder pair. In operation, an hydraulic pump pressurises the master cylinder. As the ram of the master cylinder moves, its displacement causes hydraulic oil to flow to the slave cylinder. The slave cylinder is matched to the master cylinder such that, ideally, displacement of the one is accompanied by equal displacement of the other.
It is possible, and undesirable, for the displacement of the master and slave rams to be unequal. This can occur for a number of reasons. One reason is that as the rams face a load, the hydraulic oil in the cylinders is pressurized, and will compress according to its bulk modulus of elasticity. As this occurs the master ram will tend to advance a small amount more than the slave. A second reason for unequal displacement is that there may be seepage past the seals of one or the other of the slave and master cylinders. This seepage may be very small, but over a number of repetitions a difference may build up. When a significant difference is noted it is necessary to reset the system to an equilibrium position.
A number of attempts have been made to discourage unequal lifting. In one instance a cable system has been used, with cables connected to each ram. The cables are paid out and drawn in at a common rate as the rams move. A cable system of this nature is an added expense. In another known system, shown in FIG. 1 as "Prior Art" and indicated generally as A20, the master A22 and slave A24 are connected to a dump line A26 which may be opened to supply line pressure by a by-pass valve A28, or by operation of a pilot operated check valve A30. Pilot operated check valve A30 opens when the pressure PMS in the master and slave line exceed the pressure PMC in the main cylinder beneath the master ram A32.
If the displacement of the slave and master rams in the system of FIG. 1 is unequal, the lift must be lowered to the ground for reset. With the load removed the bypass valve can be opened, and the two rams brought to their lowest, initial datum position.
The prior art embodiment indicated schematically in FIG. 1 also includes a large number of other elements. It would be advantageous to eliminate as many as those elements as possible in order to reduce the costs of the hydraulic system of a twin post lift. As a matter of pure enumeration the elements of the prior art embodiment are as follows: A34 is a motor; A35 is a hydraulic pump driven by the motor; A36 is a hydraulic reservoir or tank from which hydraulic pump A35 draws hydraulic fluid; A38 is a flow control which permits hydraulic fluid to be pumped from or dumped back into reservoir A36; A40 is a hydraulic tube assembly; A42 and A46 are both 90 degree elbows; A44 and A54 are both tees; A48 is a hydraulic tube assembly; A50, A52, A56, A60 and A62 are all adaptor fittings; A58, A66 and A70 are all hydraulic pipe assemblies; A64 is a pipe union; and A68 and A72 are hydraulic fuses.
Inasmuch as the market for twin post lifts is very competitive, and twin-post lifts of this nature are relatively inexpensive, the price of the pilot operated check valve is significant. It would be advantageous to eliminate both the cable system, the pilot operated check valve, and as much associated hardware, from the twin post lift as possible.
It is known to nest the master and slave hydraulic cylinders within the profile of the posts of a twin post lift, and to have lifting carriages driven by those nested cylinders. In the event that the hydraulic system should lose pressure while a load is still in a raised position, it is important that the lifting carriages be prevented from descending unexpectedly or uncontrolledly. To prevent this it is advantageous to have a safety stop system in which a rack, or similar device, is located on either the lifting carriage or on the upright, or post, and a corresponding pawl, gear tooth, catch, hook, dog, or other similar device is located on the other. If the load should start to slip the safety dog will engage the next available element of the rack and prevent further descent of the load.
It is advantageous to construct the major longitudinal element of the post from a roll formed channel having a back and a pair of extending legs. The legs can be formed to have a wide portion and a narrow portion, such that a lifting carriage carried within the channel is restricted to longitudinal motion along the channel. However, it is also advantageous to place the rack on the hidden spine of the lifting carriage, where it is nested inside the channel, and is not generally visible in use. Inasmuch as the channel extends outwardly from the back of the lifting carriage space, and space within the depth of the channel is restricted, it is advantageous to provide a deeper rebate to accommodate the rack while still maintaining the guide geometry of the legs of the channel. This can be achieved by locally forming the back of the channel outwardly to give a larger space for the rack. Further, by forming the back outwardly the desired result can be achieved while retaining the monolithic roll formed part. This keeps the manufacturing cost of the post down, while actually deepening the channel and increasing its resistance to bending. Finally, it is advantageous to reinforce the edges of the post closest to the lifting arms of the carriage. One way to do this is to fold the roll formed sheet over to give a double thickness locally.